1. Field of the Invention
This invention relates to high-activity solid catalysts suitable for use in esterification reactions and/or ester interchange reactions and preparation processes thereof.
2. Description of the Prior Art
Catalysts, for example, organic acids such as p-toluenesulfonic acid, inorganic acids such as sulfuric acid and hydrochloric acid, alkalies, and tetraalkoxy titaniums have conventionally been used for esterification reactions and ester interchange reactions. These reactions are widely used in the industry. For instance, useful resin plasticizers such as dioctyl phthalate (hereinafter abbreviated as "DOP") and dioctyl adipate (hereinafter abbreviated as "DOA") are produced industrially by esterification reactions. Their production is however accompanied by problems as will be described next.
Catalysts employed in these industrial esterification reactions are of course required to have practical and high activities. It is however very difficult to achieve the reactions between acids and their corresponding alcohols to approximately 100%. It may be contemplated to use these catalysts in large amounts with a view to accelerating the reaction velocities. The removal of catalyst residue gives more problem as such catalysts are employed in larger amounts. In order to improve the electrical characteristics, thermal stability and the like of plastic compositions with plasticizers incorporated therein, it is essential to remove still-remaining unreacted acids to an acid value of 0.1 or even lower. Such a low acid value is also required in various specifications. To this end, there is also an outstanding demand for high-activity catalysts which exhibit high activities even at low concentrations. In the case of p-toluenesulfonic acid catalyst, its catalytic activities are sufficiently high even at low temperatures but it is accompanied by a drawback in the removal of catalyst residue. On the other hand, alkoxy titanium catalysts are accompanied by the same drawback as acid catalysts. Namely, neutralization and water-rinsing steps are indispensable unless acid values are lowered significantly by the removal of catalyst residue (inclusive of removal of the catalysts through their decomposition). This neutralization step however leads to another problem that the remaining acid is converted to a salt and hence acts as a surfactant, thereby making it difficult to separate the aqueous mixture into an ester layer and water layer. As the acid becomes more complex, its removal becomes more difficult so that the product of the esterification reaction cannot be treated to a low acid value.
As a method for the removal of catalyst residue of a titanium catalyst, it has recently been proposed to employ a chelating agent (U.S. Pat. No. 4,506,091). Researches are also under way to provide heterogeneous solid catalysts, which permit minimized dissolution of their catalytic components in reaction systems.
For example, an esterification catalyst supported on a carrier has been proposed [Chemical Abstracts, 131919h (1984)]. Similar to conventional catalysts, this catalyst however requires removal of its acid component by washing it with an alkaline solution after conducting an esterification reaction fully. It is hence not satisfactory. As titanium catalysts of the heterogeneous system which feature minimized loss due to their dissolution in esterification reactions and hence permit their use in small amounts, there have been proposed certain catalysts including that obtained by treating a polymer of a tetraalkoxy titanate hydrolytically with a peroxide in the presence of an alkali and then treating the thus-treated polymer under reflux in a monohydric alcohol such as n-butyl alcohol or heptanol (Japanese Patent Laid-Open No. 94296/1978). They are however accompanied by one or more shortcomings that their preparation processes are complex and/or difficulties are encountered in imparting catalytic properties most suitable for various esterification reactions
In U.S. Pat. Nos. 3,194,764 and 2,643,262 referred to in the former U.S. patent, it is also disclosed to use the reaction product (glycol titanate) of an orthotitanate and glycol as a catalyst upon preparation of a mixed ester, which is useful as a synthetic lube oil, from a monohydric alcohol, dicarboxylic acid and glycol. These patent publications also disclose that the properties of the glycol titanate are in between those of monomeric glycol titanate and those of polymeric glycol titanates, it is in a liquid or solid form, and moreover, it has compatibility with the reaction system (solubility, miscibility, dispersibility). It is also disclosed that the catalytically-active component is a chelate compound with the hydroxyl groups of the glycol, which was employed upon preparation of the catalyst, coordinated about a titanium atom, although the structure of the catalytically-active component is said not to be fully elucidated. When an esterification reaction is conducted by using the above catalyst system, its catalytic activities are however not satisfactory due to its insufficient insolubilization and crosslinking degree, thereby failing to provide an esterification reaction product of a low acid value.